The continental scaup population (lesser [Aythya affinis
] and greater scaup [A. marila
] combined) has declined markedly since 1978 (Austin et al. 1998, Afton and Anderson 2001). Annual population estimates of scaup have been below the population goal of the North American Waterfowl Management Plan since 1985 and reached an all-time low in 2006 (Wilkins et al. 2006). Afton and Anderson (2001) reported that the decline in the continental scaup population likely is driven by a decline in the lesser scaup population.
The segment of the scaup population wintering in states bordering the Gulf of Mexico and migrating north along the Mississippi River valley, subsequently through Iowa, Minnesota, and North Dakota (hereafter upper-Midwest), comprises a major component of the continental population and likely is experiencing the largest decline (Afton and Anderson 2001). Located partially within the upper-Midwest, Region 3 of the U.S. Fish and Wildlife Service (2002) declared lesser scaup a conservation priority citing a need to (1) provide key information that increases our understanding of limitations to conservation of this species, (2) acquire biological information to support conservation actions relating to habitat availability and quality, and (3) provide technical assistance to initiate, augment, or redirect conservation actions so that the best management practices can be applied. Lesser scaup have high recreational and economic values; high historical abundance established this species as a staple to both hunters and bird-watchers (Chabreck 1964, Jessen 1981, U.S. Fish and Wildlife Service 2002).
Several major factors have been hypothesized as causing the scaup population decline including (Austin et al. 2000, Afton and Anderson 2001): (1) decreased quality and quantity of food resources on winter and spring migration stopover areas, (2) accumulation of contaminants, and (3) climate and habitat changes on boreal forest breeding areas, all of which may be directly or indirectly affecting female survival or recruitment. The first factor listed above has been formalized as the Spring Condition Hypothesis (Afton and Anderson 2001) and has been most extensively tested to date (Anteau 2002, 2006; Anteau and Afton 2004, 2006).
The Spring Condition Hypothesis states that reproductive success of lesser scaup has declined because females are arriving on breeding areas in poorer body condition than in the past, or not arriving at all, due to a decline in availability and/or quality of forage resources preventing females from acquiring sufficient nutrient reserves (lipid, protein, and mineral) during spring migration (Afton and Anderson 2001). Anteau and Afton (2004) documented a decrease, from historical levels, in body mass and lipid reserves of female lesser scaup currently migrating through northwestern Minnesota and arriving to breed in southwestern Manitoba. Concomitant with the decline in female body condition, Anteau and Afton (2006) also documented a decline, from historical levels, in forage quality of lesser scaup diets. Anteau (2006) further tested the scope and mechanisms of the Spring Condition Hypothesis and reported several key findings: (1) lipid reserves of females migrating through the upper-Midwest currently are much lower than are those of females at Pool 19, a major spring stopover to the south, (2) females migrating through the upper-Midwest are not storing lipids as expected, but rather are catabolizing lipids, and (3) densities of amphipods, a preferred lesser scaup food, currently are very low throughout the upper-Midwest and apparently are causing the observed decline in lipid reserves. In summary, all of the findings to date have been consistent with the Spring Condition Hypothesis, and, thus, this hypothesis presently can not be rejected.
Several important questions remain regarding Anteau’s (2006) results, one of which is, what is the relative importance of stopover areas in the upper-Midwest for accumulation of nutrient reserves by migrating female lesser scaup? Rates and distances moved during a single migratory flight may be dependent on female body condition; females in superior condition may take longer, more rapid and frequent flights. If so, females in superior condition might have a lower probability of being sampled within the upper-Midwest region. Anteau (2006) attempted to evaluate this question by color-marking lesser scaup at Pool 19, with respect to body mass, and then observing them within the upper-Midwest; however, too few observations were obtained to draw conclusions (see Appendix E in Anteau 2006:212). Clearly, Anteau’s (2006) results indicate that many females migrating through the upper-Midwest in spring are in very poor body condition, but forage conditions may be better in the southern prairies of Canada, allowing females to regain body condition there before moving on to more northerly breeding areas. Thus, quantitative information on the proportion of time females spend in each of these regions of North America during spring migration would be a useful first step in evaluating Anteau’s (2006) results and, ultimately, in prioritizing migration stopover areas for habitat conservation and management.
Accordingly, we are conducting a pilot study with the major objective of documenting spring migration corridors and migratory flight parameters of radio-marked females, in order to estimate the proportion of time individuals of varying body mass spend on identified wetlands within the upper-Midwest and prairie Canada during spring. If after departing the upper-Midwest, females do spend large amounts of time in prairie Canada before moving on to more northerly breeding areas, then future research of forage conditions and whether or not females are able to regain body condition in prairie Canada would be needed. Given the theoretical battery life of the satellite radios, we will collect information on several secondary objectives, such as fall migration corridors and affiliations of breeding and wintering areas of females. We will use the pilot study to determine whether a larger study is feasible to rigorously answer the questions of interest, and whether additional funding for such a study can be secured. Finally, we will trap and band as many lesser scaup at Pool 19 as possible, which will allow us to randomly select females of various body mass for radio-marking, but also will provide opportunity for direct estimates of harvest rate and survival, using new band analysis techniques (Nicolai et al. 2006).